There cannot be obtained diene copolymers having a superior balance between a heat resistance and workability, by means of copolymerizing non-conjugated diene compounds such as 5,5-diallyl-2,2-dimethyl-1,3-dioxane represented by the hereinafter-mentioned formula (3) having two carbon-to-carbon double bonds with olefins, in the presence of a commonly-used addition polymerization catalyst. The reason is that one of the two carbon-to-carbon double bonds is involved in not an addition polymerization reaction but an undesirable reaction such as a cross-linking reaction, although another carbon-to-carbon double bond is involved in an addition polymerization reaction, and as a result, there is obtained only a copolymer, which (i) has no cyclic structure in its main chain contributing to a heat resistance, and (ii) is not necessarily a thermoplastic copolymer contributing to workability.
On the other hand, Macromolecules, Vol. 35, pages 9640-9647 (2002) discloses a preparation method of an ethylene-cyclopentane alternating copolymer having a superior heat resistance due to a cyclic structure contained in its main chain, the method comprising the steps of (a) copolymerizing ethylene with cyclopentene by coordination ring-opening polymerization, thereby obtaining an alternating copolymer having carbon-to-carbon double bonds in its main chain, and (b) hydrogenating those carbon-to-carbon double bonds, thereby obtaining an ethylene-cyclopentane alternating copolymer. However, the above-mentioned preparation method has a problem in that (i) it is not easy to synthesize a monomer for preparing a target substituent-carrying alternating copolymer, and (ii) even if such a monomer can be synthesized, the above-mentioned hydrogenation step (b) may unfavorably hydrogenate also those substituents in an alternating copolymer obtained in the above-mentioned copolymerization step (a).